The signal path between two telephones, involving a call other than a local one, requires amplification using a four-wire circuit. The cost and cabling required discourages extending a four-wire trunk circuit to a subscriber's premises from the local exchange. For this reason, the four-wire trunk circuits are coupled to two-wire local circuits, using a device called a hybrid.
Hybrid echo, the primary source of echo generated from the public-switched telephone network (PSTN) is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire PSTN conversion points.
Unfortunately, the hybrid is by nature a leaky device. As voice signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back, creating an echo of the speech signal. Provided that the total round-trip delay occurs within just a few milliseconds, the echo results in a user perception that the call is ‘live’ by adding sidetone, thereby making a positive contribution to the quality of the call.
In cases where the total network delay exceeds 36 ms, however, the positive benefits disappear, and intrusive echo results. The actual amount of signal that is reflected back depends on how well the balance circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of signal being reflected back.
The effective removal of hybrid echo is one key to maintaining and improving perceived voice quality on a call. This has led to intensive research into the area of echo cancellation, with the aim of providing solutions that can reduce echo from hybrids. By employing the results of this research, the overall speech quality has significantly improved.
It is known in the art to employ adaptive filtering to address hybrid echo cancellation. In Normalized Least Mean Square (NLMS) adaptive filtering, adaptive filter coefficients are used to map the hybrids in the signal path. Using these coefficients, the NLMS adaptive filter cancels signal reflections from the hybrids in the signal path. To adapt and stabilize the adaptive coefficients so that the Echo Return Loss Enhancement (ERLE) is maximized for all hybrids, the adaptation step of the NLMS algorithm is varied.
Under ideal conditions, a generally acceptable convergence time requires that the echo canceller achieve 27 dB of ERLE (Echo Return Loss Enhancement) in 0.5 sec. Once the coefficients are converged, the echo is canceled from the input signal.
It is an object of an aspect of the present invention to minimize the convergence time of the adaptive filter for different hybrids in the signal path and to obtain as much ERLE possible under different line conditions.